Electrospray ionization (ESI) has been useful to study nonvolatile, thermally-labile organic and/or biomolecules by mass spectrometry. In general, ESI is carried out at ambient atmospheric pressure.
ESI sources can be viewed as providing direct ionization, such as by nanospray that reduces flow rate and capillary size, or by subambient pressure ionization with nanoelectrospray (SPIN), or by pneumatic pressure ionization such as sonic spray ionization (SSI).
ESI sources can also be viewed as providing post-ionization, i.e. ionization after desorption.
In general, ESI requires ion transport through an interface into a vacuum region where a mass analyzer and detector can be located. The interface may be a capillary tube at atmospheric pressure.
To maintain high throughput of the analyte gas, the vacuum region must be maintained at high vacuum. Thus, an electrospray mass spectrometer can have large pumps with high pumping speed. Typically, a rough pump and a turbo pump are used.
A drawback in this arrangement can be heavy air loading of the turbo pump and loss of high vacuum. In general, this means that a relatively larger pump must be used to achieve a given vacuum and certain transport characteristics in an electrospray mass spectrometer.
For example, conventional mass spectrometers may have two large throughput mechanical rotary vane pumps, as well as a large capacity turbo pump with multiple pumping stages, or multiple turbo pumps. Such instruments can have a mass of over 100 kg.
Another drawback is that to reduce the amount of gas loading in the system, it would be necessarily to reduce the throughput of analyte gas. This can greatly reduce the sensitivity of the instrument.
These drawbacks make it difficult to use ESI efficiently at low pressures or to inject analytes directly into a mass analyzer of a mass spectrometer using ESI.
There is a continuing need for a means to reduce the amount of gas loading required in an electrospray mass spectrometer so that the size of the pumping apparatus can be reduced.
There is also a need for a mass spectrometer that can inject ions by electrospray directly into a high vacuum region of a mass spectrometer to reach a mass analyzer.